Re: PMTU discovery broken in Linux for UDP/raw application if the socket is not bound to a device

[David Ahern <dsahern@gmail.com>]

[ adding netdev so others know ]

On 10/9/18 3:38 AM, Preethi Ramachandra wrote:
> Hi David,
> 
> I tested your fix, Linux is updating PMTU successfully. 

ok, I'll send a formal patch


> 
> Thanks,
> Preethi
> 
> On 10/7/18, 8:59 AM, "David Ahern" <dsahern@gmail.com> wrote:
> 
>     The correct mailing list is netdev@vger.kernel.org (added)
>     
>     non-text emails will be rejected.
>     
>     
>     On 10/3/18 10:15 PM, Preethi Ramachandra wrote:
>     > Hi,
>     > 
>     >  
>     > 
>     > While testing the PMTU discovery for UDP/raw applications, Linux is not
>     > doing PMTU discovery if the UDP server socket is not bound to a device.
>     >  In the scenario we are testing there could be multiple VRF devices
>     > created and an application like UDP/RAW can use a common socket for all
>     > vrf devices. While sending packet IP_PKTINFO socket option can be used
>     > to specify the vrf interface through which packet will be sent out. In
>     > this case, when packet too big icmp6 error message comes back to Linux
>     > on a vrf device, a route lookup is done on default routing-table(0) for
>     > src/dst address which case, the route will not be found and packet is
>     > dropped. If the route lookup happened with proper VRF device (packet’s
>     > incoming index), the route lookup succeeds,  PMTU discovery is successful.
>     > 
>     >  
>     > 
>     > This might need a fix, please take a look.
>     > 
>     >  
>     > 
>     > *Linux version *
>     > 
>     >  
>     > 
>     > Linux  4.8.24
>     > 
>     >  
>     > 
>     > *Code flow *
>     > 
>     >  
>     > 
>     > Linux code where it expects socket’s bound device in order for PMTU
>     > discovery to happen.
>     > 
>     > *void ip6_sk_update_pmtu*(struct sk_buff *skb, struct sock *sk, __be32 mtu)
>     > 
>     > {
>     > 
>     >                     struct dst_entry *dst;
>     > 
>     >  
>     > 
>     >                     ip6_update_pmtu(skb, sock_net(sk), mtu,
>     > 
>     >                                                             
>     > sk->sk_bound_dev_if, sk->sk_mark, sk->sk_uid);*<<<<< This is the point
>     > where it expects socket’s sk_bound_dev_if to be set. In our testing this
>     > is actually 0, since the socket is not really bound to a vrf device.*
>     
>     Try this based on top of tree for 4.19-next (whitespace damaged on paste
>     so you'll need to manually apply and handle differences with 4.8):
>     
>     diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>     index 6c1d817151ca..50b95b48b911 100644
>     --- a/net/ipv6/route.c
>     +++ b/net/ipv6/route.c
>     @@ -2360,10 +2360,13 @@ EXPORT_SYMBOL_GPL(ip6_update_pmtu);
>     
>      void ip6_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, __be32 mtu)
>      {
>     +       int oif = sk->sk_bound_dev_if;
>             struct dst_entry *dst;
>     
>     -       ip6_update_pmtu(skb, sock_net(sk), mtu,
>     -                       sk->sk_bound_dev_if, sk->sk_mark, sk->sk_uid);
>     +       if (!oif && skb->dev)
>     +               oif = l3mdev_master_ifindex(skb->dev);
>     +
>     +       ip6_update_pmtu(skb, sock_net(sk), mtu, oif, sk->sk_mark,
>     sk->sk_uid);
>     
>             dst = __sk_dst_get(sk);
>             if (!dst || !dst->obsolete ||
>     
>     
>     
>